1. Field of the Invention
This invention relates generally to thermal dispersion sensors and more particularly to such sensors employing thermally sensitive detection elements mounted externally to the fluid container or conduit to sense liquid level therein or mass flow rate therethrough. Alternatively, an insertion device can also be employed wherein the detectors are inside the vessel but, because of various sealing mechanisms, are still topologically outside the container and isolated from the media.
2. Discussion of the Related Art
In many industrial and commercial fields there is a requirement for compact and versatile flow rate detectors which positively determine that a particular mass of fluid is flowing, has stopped flowing, is flowing above or below a predetermined threshold mass velocity level or the actual mass flow rate at which it is flowing. Alternatively, such a device may be used to determine when the level of rising or falling liquid in a container has reached a predetermined height. The conduits and containers involved may be oriented vertically, horizontally, or they may be inclined, and can range in size from fractions of an inch to as much as several feet in diameter. Returning to the mass flow rate sensing application, this requirement is particularly strong in manufacturing situations where it is critical that the amount and velocity of flowing gas must be known. This is even more critical where those gases are toxic, which often occurs in the manufacture of electronic chips.
Fabrication methods incidental to the mechanical operating principles of current devices used in the electronic chip industry often result in dead-end cavities, labyrinthian passageways, irregular and rough surfaces from welding, close diametral clearances between moving parts. The impossibility of purging and other effects from the labyrinthian passages are also detrimental to delivering clean gas. In many instances, the absence of polished surfaces, the presence of close fitting parts and dead-end cavities can all but prevent the delivery of pure products or the purging of the system when gaseous products are changed. Moreover, current devices used in chip manufacturing typically employ moving parts. Inevitably, particle matter generated by the moving parts further contaminate the gaseous media being employed. Ultra cleanliness and purity are absolutely necessary if high quality electronic chips are to be manufactured. In many instances, smooth finishes and the ability to effectively clean the conduit can be all but impossible to achieve with some devices currently on the market. Failure to note that flow has ceased or has been reduced below or increased above certain predetermined flow velocity thresholds in a conduit may be very costly and in some instances could be catastrophic. The same can be true of liquid levels. As processes increase in speed and output, and precision becomes ever more of a requirement, often resulting from advances in technology, such failures tend to be ever more costly and dangerous.
Devices have long been available for detecting and, in some cases, measuring the rate of flow of fluids or liquid level. A common type of flow detector utilizes the force exerted by the moving fluid against a paddle or movable wall immersed in the fluid or the fluid flow to indicate or determine the rate of fluid motion. Regardless of the form chosen for the immersed object, for example, propeller, vane, piston, deflection arm, drogue or the like, all of these devices are subject to a number of potentially serious shortcomings for certain uses, especially for mass flow rate measurements as required in chip manufacturing and most other applications. All of the above require compensation for pressure and temperature and the effects these variables have on density of gaseous media. Moveable parts tend to deteriorate after continued immersion for extended periods of time and can become corroded or frozen in place after even brief contact with many fluids. This is especially true with gases or liquids which may be toxic, corrosive, or both. Seals and packing, always at least minor problems, become monumental tasks when moving parts are involved. Mechanical deformation and fatigue induced breakdowns also plague this class of indicators. When these mechanical devices are used, they are, by and large, wholly unsuitable for chip manufacturing purposes as well as many other exacting purposes. They are also generally unsuitable for the detection of flow stoppage, reduction in flow velocity below a predetermined level, or changes in fluid level in customary commercial and industrial applications. This subject will be further discussed below.
Because of the disadvantages of requiring the force exerted by the moving fluid against some object in order to provide detectable motion or level changes, thermal dispersion mass flow meters have become a common choice for flow metering devices in the commercial and industrial metering and level sensing markets. A typical flow sensor element for use in such meters is the resistance temperature detector (RTD), the resistance of which is related to the temperature of the element itself. Although RTDs are the preferred device, many other types of small heatable temperature detector/heaters could also be considered. Thermocouples, thermistors, temperature sensitive diodes and other transistors or solid state devices could be used. Also RTDs come in many forms such as chips, wire wound elements and grids. A typical flow rate or level sensor employs at least two RTD elements. One of them is referred to as a reference element and is normally unheated. The active RTD element is heated and the temperature reduction effect of mass flow or wetting on the heated element provides a measure of the mass flow velocity or a phase change from dry to wet of the substance in the conduit or vessel being monitored. The density of a gaseous fluid flowing across the active RTD is also a directly proportional factor in the amount of heat dissipated from the RTD. As discussed above, RTD sensors can also be employed for liquid level detection and interface detection of gas to liquid, and non-miscible liquids such as oil/water, clear water and sludge or slurries, to name a few.
There are many configurations of dispersion mass flow sensors, and more particularly, of heated RTD type sensors. An early such flow detector is found in U.S. Pat. No. 3,366,942. This patent discloses a reference sensor, a heated or active sensor, and a separate heating element located closely adjacent the heated sensor element. The basic principle of operation of dispersion flow meters is well known and is discussed in this patent. A different configuration of a three-element thermal dispersion sensor is shown in U.S. Pat. No. 4,899,584. There any many other examples of detectors employing differential temperature sensors, some having three elements as described in the patents mentioned above, and some having two elements, where the active sensor has the heater integral therewith and is self heated. Even a single element differential temperature sensor may be employed. The single element sensor works on a time sharing basis where it acts as a reference sensor part of the time and is then heated to act as the active sensor, switching alternately in relatively rapid succession. Another example of a differential temperature sensor is shown in U.S. Pat. No. 5,780,737.
The devices shown in the patents mentioned above have no moving parts and have proven satisfactory, at least in many circumstances where it is desirable to determine that fluid flow has stopped. They are also very sensitive to low levels of mass flow of fluid. It is important to note that in the examples above and in many other related examples, the RTD type sensors are mounted in a thermal well and are immersed in the fluid flowing in a conduit, or are positioned to be wetted by liquid at predetermined levels.
For the manufacture of electronic chips, where toxic gases are employed, immersion sensors of any type are generally not appropriate because of the intrusion of the expensive thermal wells into the relatively small conduit containing the flowing stream. Such devices typically are too large for the conduits involved in electronic chip manufacture and likely cannot be properly purged of possible residual gases from previous uses.
For example, in electronic chip manufacturing, noxious and often toxic gasses are used in vapor deposition. In order to control the flow of those vapors and to ensure that excess vapors do not overload the system""s capacity to properly contain them, an excess flow sensor and switch can be used. Examples of prior art devices which can be employed for such purposes are flow rate magnet/reed switches. Magnetic switches of this type are sold by Nupro Company under the designation xe2x80x9cFV4 Series Vertical Flow Sensor,xe2x80x9d and the series xe2x80x9cAP74 Vertical Flow Switchxe2x80x9d is sold by Advanced Pressure Technology, specifically for use in the manufacture of electronic chips. An actual switch is required, which is external to the conduit through which the gases flow, and a moving magnet is positioned within the fluid conduit. Thus these devices are partially direct contact and partially remote sensor devices. These mechanical switches are not well suited for mass flow rate sensing because they are sensitive to volume flow rate and mass flow rate errors are introduced because of density variables. Principally, pressure is the primary cause of such density variables. For these types of switches, several different models would be required to satisfy the various trip points that might be specified by any user. The trip point of choice is fixed and set in the factory and is not accurate as explained in their specifications.
Temperature can also affect density and is a contributing error factor in some cases. Trade literature for such magnetic switch products show that the trip point flow rate is a function of pressure. One model will trip at 15 SLPM (standard liters per minute) when the system is pressurized to 100 psig. If this model were placed in a 20 psig system, it would trip at 7 SLPM. That is more than a 100% difference from a trip point of 15 SLPM. Thus the requirement of many different models for different flow rates and density uses. Ideally, the customer would prefer to have a single switch with a trip point of, for example, 10 SLPM, which would trip at that value over any pressure range between 0 and 100 psig. Also, these magnetic switches have a wide hysteresis where the trip point has a very different value than the reset point of a particular switch.
Not only is it all but impossible to achieve appropriate cleaning and smooth finishes, but welding, purging and other effects from the labyrinthian passages of the magnetic sensors discussed above can be detrimental to the delivery of clean gas. Additionally, the moving magnet and its enclosure in the flowing media may also generate foreign particles which could contaminate the electronic chips being manufactured.
There are some external or conduit surface mounted temperature detectors previously available. An example is the series AP7300 Flow Switch by Advanced Pressure Technology and the Rheotherm Flow Instruments of Intek, Inc. These are indeed external surface mounted devices but there is no indication of the existence of a local, small, specially prepared surface to increase the sensitivity of the sensing element, at low power levels, to the rate of flow of the fluid within the conduit to which they are attached. Additionally, a separate heater is employed and miniaturization by use of a chip type RTD is not shown. Another example is the use of wire RTDs wrapped around small and medium size tubes as the sensor element. Such a sensor would be relatively large in area and require high power for heating. If the conduit is thinned on its periphery to bring the wire wound RTD closer to the flowing medium, it would compromise the mechanical strength of the tube. Power requirements can be relatively high and sensitivity may be insufficient for detecting small flow rate changes, especially when the mass flow rate is low.
It is readily understood that whenever the substance being measured is in direct contact with the measuring instrument, the measuring instrument will have some effect upon the substance being measured. Thus, there is a need for a sensitive detector for liquid level or fluid flow to accurately and sensitively measure fluid flow or level without directly affecting or being in direct contact with the fluid being measured. A general purpose industrial thermal flow switch employing thermal wells, such as the Model FLT-93S manufactured and sold by Fluid Components Intl, is generally inappropriate for the particular application (electronic chip manufacturing) to which one example of the present invention is most specifically directed. Further, such devices are inappropriate for most other purposes of the invention described below because they have relatively large heaters and consequently require relatively high operating power. Additionally, those devices do not employ laminar boundary layer flow sensing.
This invention relates generally to an extremely sensitive, self heated, miniaturized, low power, rugged, chip-type resistance temperature detector (RTD) and a special mounting therefor on the fluid flow conduit wall, or inserted into a vessel, but external to the flow path in that conduit or the media in the vessel. Since the invention can be employed to sense mass flow in a conduit or liquid level in a container or stand pipe, whenever either term xe2x80x9cconduitxe2x80x9d or xe2x80x9ccontainerxe2x80x9d is used herein, it should be understood to include the other term as appropriate.
The invention in the preferred form employs a chip RTD designed as a temperature sensor, which is mounted with its flat side against a specially created flat surfaced, thin walled small receptacle in the exterior of the fluid conduit or container. The chip may be self heated to deliberately cause it to sense a higher temperature than it would if used in its usual capacity as a temperature sensor. The conduit is formed with a thin, local small flat area which receives the chip RTD in close proximity but not actually in the flowing fluid. The thin-walled small flat area or detent formed in the external surface of the fluid conduit is configured so that the heat generated in the heated chip will flow principally and immediately to and be carried away by the media, that is, to the fluid, rather than axially or circumferentially along the conduit or container wall. The small size and high resistance of the chip RTD permits operation at a high chip temperature and at low power and low current to accommodate the modest power available from an intrinsically safe 4-20 mA single wire loop and without grossly compromising the structural integrity of the vessel or duct by requiring a large local thin area that likely would be needed for ordinary contemporary high-power-consuming larger devices as discussed previously.
The instrument industry has developed a standard practice of operating sensors with a low voltage (less than 24 VDC) and a low current in the range of 4-20 mA over a two-wire loop powered system in order to make them intrinsically safe. xe2x80x9cIntrinsically safexe2x80x9d means, in effect, that a short circuit spark from any postulated failure will not set off an explosion when that spark occurs in an explosive atmosphere. No other thermally activated flow rate, interface or liquid level sensor is known which can operate under these low power, intrinsically safe conditions.
The configuration of the invention permits operation without a bulky transformer when 110 volt or higher AC voltage is the only available source of power. At these lower power needs, a simple dropping resistor can serve to reduce the voltage without excessive heating. High voltage DC can also be easily accommodated by also using a dropping resistor. The combination of small size, self heating, and high resistance of the heated sensor, and the mounting preparation of the conduit or container wall improve on previously known methods to the point where the extremely accurate sensor of this invention can operate on the 4-20 mA loop or from a small, economical power supply where no other power is available. Moreover, because the device senses boundary layer flow, it can easily accommodate higher flow rate ranges than can be accommodated by, for example, small insertion-type sensors immersed in turbulent flow fields and conditions in the usual thermal well. This concept can be applied to thermal well sensors of the type of model FLT-93S (sold by Fluid Components Intl) but that is not the preferred form.
In this particular configuration, a thin wall of pipe is left after a flat surface is prepared to mount the flat side of the chip RTD in the most intimate contact possible with the media, forming a non-intrusive flow sensor in the preferred version. But the same methods can be employed with equally favorable result with an insertion device. Even though it projects into the fluid flow path, the insertion device is topologically equal to the preferred mechanism where the RTD chip is outside the vessel and with sufficient flat area exposed to the flow field so that boundary layer flow occurs locally on the flat surface of the insertion instrument.
The invention also contemplates a method for making and using the sensor/conduit wall combination, the use of a single chip RTD which ultimately acts as the reference sensor and the heated sensor in a time sharing arrangement, and possibly using several such chip RTD sensors around a larger pipe to compensate for flow stratification, or on a vertically oriented still well at various vertical positions for level gauging. The term xe2x80x9clarger diameter pipesxe2x80x9d could be applied to pipes ranging from about one inch in diameter to any practical size.
One or more sensors could be used to detect both flow rate and phase change where, for example, a horizontal liquid-handling pipe leads to a pump inlet. The device can sense the liquid flow and, should gas appear, the instrument can signal the presence of gas as well as liquid flow rates. In the last example, three RTDs might be employed wherein two RTDs are heated and one RTD would be used for a reference sensor. One heated RTD would be mounted on the top of the horizontal-flow inlet pipe to signal wet/dry and the second heated RTD would be located where it would sense a wet and flowing condition.
As referred to above, customary industrial and commercial application are other important uses of the invention. In many applications where mechanical floats or immersed paddles have been unsuccessfully employed, the subject of this invention can easily be substituted for the failure prone mechanically actuated switches wherein two wires are already available for the conversion to a 4-20 mA-loop-powered thermal flow switch.
Owing to the small size of the chip-type RTD, and partially from the self-heating of the heated RTD and the preparation of the thin-walled mounting surface, very low levels of heater power may be exclusively employed in order to generate a sufficiently high differential temperature between the heated RTD and its companion xe2x80x9creferencexe2x80x9d RTD.
The two wires already installed to signal contact position of the prior mechanically actuated switches can be employed in the present invention in the case of a retrofit. Customarily and widely used in industrial instrumentation is a two wire loop carrying from 4 to 20 milliamperes. This power supply can be attached to one end of the two wires already available and the electronics and sensing RTDs can be attached to the wires at the opposite end where the mechanical switches had been located.
To the exclusion of other similar thermal switches that consume much higher power levels, 10 milliamperes of the loop power or less could be employed to operate the electronics and heat the heated sensor. The remaining 10 milliamperes can be variably consumed or xe2x80x9csunkxe2x80x9d to convey a signal of varying flow rate. Alternatively, a wet/dry condition could be signaled by monitoring the current in the loop. For example, 10 milliamperes could signal a xe2x80x9cdryxe2x80x9d condition and when the RTDs are wetted, an additional five or ten milliamperes could be drawn (or sunk) to signal a phase change from xe2x80x9cdryxe2x80x9d to xe2x80x9cwet.xe2x80x9d
Thus, by the teaching of this invention, it is possible to exclusively adapt thermal switches or even flow rate transmitters to the 4 to 20 mA signal/power used for modern industrial instrumentation practice. This is the case for new installations as well as retrofitting inefficient or inoperative mechanical installations wherein two suitable wires are already in place, without the need for new and expensive wiring or the necessity to undergo a weight penalty in aerospace installations. The rule of thumb is that each wire adds two pounds to the total weight in aircraft, so reduction of the number of wires used can be important. No other thermal switch is known to be able to operate on the two-wire loop, 20 mA-powered circuit. It is noteworthy that other thermal switches require higher power levels requiring expensive electrical conduits and furthermore, lack the inherent, intrinsically safe feature provided by this invention. xe2x80x9cIntrinsically safexe2x80x9d is an industrial term for a device powered at such a low level that it could not initiate an explosion from the worst spark possible in the presence of an explosive gas/air mixture. The term xe2x80x9c4-20 mA loopxe2x80x9d is used generally herein, but the current could be as high as 25 mA to signal a special condition such as a failure. The current could also be as low as 0 mA to signal other conditions.